SUMMARY-PROJECT 1-GOLDRATH: Long-lived memory cells provide protection from reinfection and can serve as endogenous defenders against tumor growth and metastases. Tissue-resident memory T cells provide essential sentinel protection at body surfaces such as the intestinal epithelium, and are now clearly understood to be among the key `first responders' in many infection settings. Although we know that resident-memory cells are an essential component of immune memory, little is known about the transcriptional pathways regulating their formation, survival and function. Improving our understanding of these topics will allow us to harness the immediate protective capacity of this vital memory T cell population and modulate their activity in the context of immunopathology, which is the overarching goal of the Program Project. Benefiting from the combined expertise in cutting-edge epigenetic and genomic analyses, CD8+ T cell biology, and chronic infection of the Program Project and Core Leaders, Project 1 will define the relationship of transcriptional programs driving unique memory states with a focus on resident versus circulating memory populations. The heterogeneity, gene-expression programs, functional activity and regulatory elements involved in resident- memory cell development will be studied to generate an integrated understanding of how transcriptional regulators such as Blimp1, Bcl6, T-bet, and E/Id proteins drive divergent differentiation programs in memory cell precursors to promote differentiation of distinct memory fates. To this end, specifically, we will: (1) Define the relationship of transcriptional programs driving unique memory states to understand how resident-memory T cell differentiation diverges from circulating memory T cell populations. Single-cell analysis of gene expression will be paired with high-throughput functional screening to assess the hybrid transcriptional network regulating the formation and homeostasis of Trm populations. (2) Decipher the contradictory dependence of Trm on the antagonistic transcriptional repressors, Blimp1 and Bcl6 in Trm differentiation and homeostasis. (3) Resolve the role of E protein transcription factors and their regulators in the transcriptional network governing the development, function and homeostasis of tissue-resident memory T cells in acute and chronic infectious settings. Our studies identifying key molecular determinants and transcriptional programs that control resident- memory T cell fate specification are critical to inform the rational design of the next generation of vaccines that will specifically aim to invoke tissue-resident memory cell-mediated protection from infectious diseases and malignancy. !